1. Field of the Invention
This invention relates to the protection of electrical circuits from overcurrents, for example from overcurrents caused by equipment faults or transient overcurrents caused by lightning, electrostatic discharge, equipment induced transients or other threats.
2. Introduction to the Invention
Many electrical circuits, for example telephone systems and other information distribution systems, are subjected both to transient overcurrents and to overcurrents of long duration for instance due to equipment failure or short circuits caused by damage to equipment. In such cases it would be desirable to incorporate in the system a device that would protect the system against both types of overcurrent but would automatically allow the system to continue functioning after a transient overcurrent has passed.
One such arrangement is described in Australian Patent Application No. 48128/85 to Glynn et al in which a pair of switching transistors in Darlington configuration is connected in series with a circuit line, the base of which is controlled by a silicon controlled rectifier (SCR) that senses the voltage drop across a resistor in series with the switching transistor. In addition, resetting circuitry is provided to reset or attempt to reset the switching arrangement periodically in the event that it trips. Another overcurrent protection circuit that will reset itself into normal operation after a transient overcurrent is described in U.S. Pat. No. 4,202,023 to Sears. However, both these circuits have a number of drawbacks. For example, the presence of a series resistor adds to the voltage drop across the device in use and will increase the difficulty and cost of manufacturing the arrangement in integrated circuit form because the resistor will have to carry load current in normal use. Also, both circuits will attempt to reset themselves indefinitely when the system is subject to a long-duration overcurrent such as caused by equipment failure with the result that it may be necessary to switch the system off before the fault can be repaired. Furthermore, in the case of the Glynn et al circuit, when the arrangement has tripped into its OFF state there will remain a relatively high leakage current through the SCR in the order of 10 to 20 mA.
Thus, according. to the invention there is provided an overcurrent protection arrangement, which comprises a switching circuit that is intended to be series connected in a line of the circuit to be protected and which will allow normal circuit currents to pass but will open when subjected to an overcurrent, the arrangement including a pulse generator which, when the switching circuit has opened, will generate pulses to a predetermined finite maximum number or for a predetermined time that reset, or attempt to reset, the switching circuit to its conducting state, the pulse generator and any other components of the arrangement taking their power supply from the voltage difference across the switching circuit, optionally after appropriate voltage regulation, for example by means of a Zener diode.
The invention has the advantage that the number of pulses that is generated in order to reset or to attempt to reset the arrangement, or the time for which they are generated, is limited so that, for example, in the case of equipment failure the source is not continually switched into the faulty equipment. Thus, the protection can discriminate between transients and persistent system faults. In the case of an overcurrent the switching circuit will rapidly switch off and will then reset itself or attempt to reset itself one or more times in case the overcurrent is due to an externally induced transient. However, if the overcurrent persists, for example if it is caused by a fault in the load circuit, the switching circuit will immediately revert to its OFF state as soon as the resetting pulse ends. Once this has occurred for the predetermined number of pulses the arrangement will remain in its OFF state indefinitely.
The switching circuit preferably comprises a switching transistor that is intended to be series connected in the circuit line, and a control transistor that determines the base or gate voltage of the switching transistor, and whose base or gate voltage depends on the voltage drop across the switching circuit. For example, the control transistor may form one arm of a voltage divider which spans the switching transistor and which sets the base or gate bias of the switching transistor, the control transistor being connected in parallel with the base and emitter or gate and source of the switching transistor. The base or gate bias of the control transistor may also be determined by a voltage divider that spans the switching transistor. In normal operation of this form of switching circuit, when no current passes along the circuit line both the switching and the control transistor are off. As the voltage on the line increases the base or gate forward bias of the switching transistor rises due to the relatively high resistance of the control transistor in its off state, until the switching transistor turns on. In normal operation the arrangement will allow the circuit current to pass with a small voltage drop across the switching transistor of about 1.5 V in the case of an enhancement mode MOSFET or about 0.65 V in the case of a single bipolar junction transistor.
When the line is subjected to an overcurrent, the voltage drop across the switching transistor increases, hence the base or gate forward bias of the control transistor increases until the control transistor turns ON, thereby shorting the base and emitter or the gate and source of the switching transistor and turning the switching transistor OFF. As this occurs the voltage across the switching transistor increases, so increasing the forward bias of the control transistor base or gate and locking the arrangement in the OFF state even if the overcurrent transient passes.
This form of circuit has the advantage that it does not require any series resistor to be provided in the line of the electrical circuit for determining the existence of an overcurrent, so that the voltage drop across the switching circuit is solely due to the collector-emitter or drain-source voltage drop of the switching transistor. In addition, the absence of a series resistor reduces the number of load current carrying components which allows easier integration of the device.
If the switching circuit has this configuration, the pulse generator is preferably arranged to short the base and emitter or gate and source of the control transistor, thereby turning it OFF which in turn will turn the switching transistor ON. This may be achieved by providing a resetting transistor for xe2x80x9cshortingxe2x80x9d the base and emitter or gate and source of the control transistor, the base or gate voltage of the resetting transistor being taken from the pulse generator.
Another form of switching circuit may be provided by a transistor switch that controls the circuit current and has a control input, and a control arrangement that controls the voltage of the control input and is responsive to an overcurrent through the switch, the control arrangement comprising a comparator circuit that compares a fraction of the voltage across the switch with a reference voltage and opens the switch if the fraction is greater than the reference voltage.
This arrangement has the advantage that it enables much flatter performance variations with respect to temperature to be obtained. In addition, it is possible to run the circuit protection arrangement according to the invention at considerably higher circuit currents without the danger of it tripping under the normal circuit current. In many cases the arrangement can be operated with up to 80% of the trip current without danger of it tripping.
Preferably the comparator circuit is powered by the voltage drop that occurs across the transistor switch, thereby obviating the need for any separate power supply.
The simplest form of arrangement may comprise a comparator circuit, for example in the form of an open loop operational amplifier, having one input terminal that is connected to a voltage reference and another terminal that samples the voltage difference across the switch by means of a voltage divider. The voltage reference should have a relatively temperature stable performance, preferably having a temperature coefficient of not more than xc2x10.5% Kxe2x88x921, more preferably not more than xc2x10.2% Kxe2x88x921 and especially not more than 0.1% Kxe2x88x921. Normally a Zener diode or band gap device will be employed as the voltage regulator.
The pulse length, separation and number will all depend on the application. Typically a pulse of up to 15, and preferably up to 250 ms will be generated, with a pulse separation of 1s to 1 hour. The arrangement will normally incorporate a pulse generator that generates a small number of resetting pulses before stopping, for example up to 10, and especially up to 3 pulses. In many devices it may be desirable for the pulse generator to generate a single pulse only before stopping, so that the protection arrangement can distinguish between a transient in the line and an overcurrent that is due to a fault in the load circuit.
Where the arrangement is intended to be employed with a.c. circuits, the series switching arrangement will be connected to the line via a rectifying bridge circuit. Alternatively a pair of equivalent circuit protection arrangements according to the invention may be employed, the two arrangements handling different cycles of the a.c. signal. This arrangement has the advantage that the voltage drop across the bridge diodes can be removed or reduced.
The overcurrent protection arrangement according to the invention may employ bipolar transistors and/or field effect transistors. Where bipolar transistors are used they are preferably used in a Darlington configuration as the switching transistor in order to reduce the base current required when the transistor is switched ON. This base current must be supplied via a resistor connected between the base and collector of the switching transistor. When the circuit switches to its blocking state the switching transistor base current is diverted through the control transistor (which is now ON) and becomes a leakage current. However, since the voltage drop across the resistor is much higher when the arrangement is in is blocking state, the linkage current is larger than the switching transistor base current. If a Darlington pair or triplet is employed as the switching transistor, the effective d.c. current gain will be increased considerably so that a much higher resistance can be used.
Where field effect transistors are employed, MOSFETS are preferred, especially enhancement mode MOSFETS. The arrangement may be produced as an integrated circuit, in which case the resistors employed in the switching circuit (and in the pulse generator circuit) may be provided by MOSFETs, for example with their gates and drains connected as in NMOS logic. Alternatively, the control transistor and the resistor which together form the voltage divider for the base or gate of the switching transistor may be provided by a complementary n-channel and p-channel pair of FETS connected in the manner of CMOS logic.
If desired the circuit may include a light emitting diode or other means for indicating that the circuit has switched.
Any of a number of means may be used to generate the pulses. Especially where a large number of pulses is intended to be generated, for example they may be generated by an astable oscillator known per se. In order to provide a sufficient time delay between the pulses, it may be appropriate for the pulse generator to include a divider, for example a counter or shift register, whose input is supplied by a relatively fast oscillator, eg. a crystal device or other circuit. Indeed, it may be possible for the user to specify the pulse frequency by selecting the divider output that attempts to reset the switch. The output of the divider will normally be fed to the comparator input via a high pass filter although a monostable oscillator could be used.
The arrangement according to the invention may be formed using discrete components or it may be formed monolithically using well known techniques. Preferably the arrangement is made in monolithic integrated form as such devices are less expensive and are also smaller and more reliable. The use of a divider as described above has the advantage that the value of any capacitors in the pulse generator circuit may be significantly smaller than those that would be required in the absence of the divider, thereby making the circuit more suitable for monolithic integration.
It is preferred for the arrangement to include no resistive components in series with the transistor switch. Such an arrangement not only reduces the voltage drop along the line of the circuit, but, more importantly, reduces the area of silicon that need be employed in an integrated circuit design of the arrangement, thereby reducing the cost.